1. Field of the Invention
The present invention relates to a nitride-based semiconductor element, and more particularly, it relates to a nitride-based semiconductor element having a nitride-based semiconductor layer.
2. Description of the Background Art
A technique of growing a nitride-based semiconductor layer, consisting of a material different from that of an underlayer, on the underlayer is known in general. In crystal growth of GaN which is one of nitride-based semiconductors, for example, a nitride-based semiconductor layer is hetero-grown on a heterogeneous substrate such as a sapphire substrate due to a small number of lattice-matching substrates. In relation to this, generally known is a technique of inserting a buffer layer grown under a low temperature between the substrate and the GaN layer in order to grow GaN having excellent crystallinity with a small number of crystal defects.
Also when the aforementioned low-temperature buffer layer is employed, however, defects of about 1xc3x97109 cmxe2x88x922 are still present and the density of reducible defects is limited. Further, it is difficult to reduce the density of dislocations. To this end, generally proposed is a technique employing an underlayer for reducing the density of dislocations by epitaxial lateral overgrowth (ELOG) when growing GaN. This epitaxial lateral overgrowth is disclosed in Journal of Oyo Denshi Bussei Bunkakai, Vol. 4 (1998), pp. 53 to 58 and 210 to 215, for example.
In this ELOG, a mask layer is formed on a prescribed region of the underlayer for selectively growing a GaN layer from the underlayer through the mask layer, so that the GaN layer grows in the vertical direction and then grows in the lateral direction. Dislocations are laterally bent due to the lateral overgrowth, whereby the density of dislocations reaching a flattened surface of the nitride-based semiconductor layer is remarkably reduced as compared with the underlayer from about 1xc3x97108 cmxe2x88x922 to less than about 1xc3x97106 cmxe2x88x922.
A method referred to as PENDEO, disclosed in International Workshop on Nitride Semiconductor (IWN 2000), Nagoya, p. 79, is also known as a method of reducing the density of dislocations through selective lateral growth. According to PENDEO, a nitride-based semiconductor layer is grown on a substrate and the surface of this nitride-based semiconductor layer is recessed to partially expose the substrate. The nitride-based semiconductor layer is laterally grown on projection portions of the nitride-based semiconductor layer. Further, Japanese Patent Laying-Open No. 2001-168042 or Jpn. J. Appl. Phys. 40 (2001) L583 discloses a method of laterally growing a nitride-based semiconductor layer on a substrate with recess portions.
In each of the aforementioned conventional methods employing selective lateral growth, however, the nitride-based semiconductor is frequently cracked in a subsequent process step such as a step of separation of the substrate into elements due to the difference between the thermal expansion coefficients of the mask layer and the grown nitride-based semiconductor or voids resulting from facets bonded on a portion of the mask layer.
In the conventional selective lateral growth, since the density of dislocations is reduced in the vicinity of the surface of the nitride-based semiconductor layer, it is difficult to obtain lattice relaxation by dislocations. Therefore, the nitride-based semiconductor is frequently cracked and the substrate is frequently warped.
Particularly when the nitride-based semiconductor layer has a smaller lattice constant than the substrate as in an AlGaN layer formed on a GaN substrate, for example, or the nitride-based semiconductor layer has a larger thermal expansion coefficient than the substrate such as an Si substrate or an SiC substrate, the aforementioned cracking or warpage is frequently caused.
An object of the present invention is to provide a nitride-based semiconductor element capable of preventing a nitride-based semiconductor from cracking and reducing the degree of warpage of a substrate.
Another object of the present invention is to effectively prevent a nitride-based semiconductor layer formed by selective lateral growth from cracking in the aforementioned nitride-based semiconductor element.
In order to attain the aforementioned objects, a nitride-based semiconductor element according to a first aspect of the present invention comprises a first region formed on a prescribed region of a substrate and provided with an element including a first nitride-based semiconductor layer having a prescribed thickness and a second region formed on a region of the substrate other than the first region and provided with the first nitride-based semiconductor layer with a thickness smaller than the thickness in the first region.
The nitride-based semiconductor element according to the first aspect is formed with the second region provided with the first nitride-based semiconductor layer with a thickness smaller than the thickness in the first region provided with the element as described above, whereby strain easily concentrates to the second region provided with the first nitride-based semiconductor layer with the smaller thickness. Thus, strain of the first region provided with the element is relaxed, whereby the first nitride-based semiconductor layer provided on the first region can be effectively prevented from cracking, and the degree of warpage of the substrate can be reduced. Consequently, the nitride-based semiconductor element can be formed on the first region with an excellent yield.
In the aforementioned nitride-based semiconductor element according to the first aspect, the first region provided with the element preferably includes a first portion, formed on the substrate, having difficulty in allowing growth of the first nitride-based semiconductor layer and a second portion, formed on the substrate, easily allowing growth of the first nitride-based semiconductor layer, and the second region preferably includes a third portion, formed on the substrate, having difficulty in allowing growth of the first nitride-based semiconductor layer. According to this structure, the thickness of the portion of the first nitride-based semiconductor layer formed on the second region can be easily reduced below the thickness of the portion of the first nitride-based semiconductor layer formed on the first region.
In the aforementioned nitride-based semiconductor element including the first, second and third portions, the first portion of the first region is preferably formed with a first width, and the third portion of the second region is preferably formed with a second width larger than the first width. According to this structure, the first nitride-based semiconductor layer is more difficult to be grown on the third portion of the second region having a larger width as compared with the first portion of the first region having a smaller width, whereby the thickness of the portion of the first nitride-based semiconductor layer formed on the second region can be easily reduced below the thickness of the portion of the first nitride-based semiconductor layer formed on the first region.
In the aforementioned nitride-based semiconductor element including the first, second and third portions, the first portion of the first region having difficulty in allowing growth of the first nitride-based semiconductor layer preferably includes any of a region having no buffer layer, a region having a mask layer and a region having a recess portion, the second portion of the first region easily allowing growth of the first nitride-based semiconductor layer preferably includes any of a region having a buffer layer, a region having no mask layer and a region having no recess portion, and the third portion of the second region having difficulty in allowing growth of the first nitride-based semiconductor layer preferably includes any of a region having no buffer layer, a region having a mask layer and a region having a recess portion. According to this structure, the first portion easily allowing growth of the first nitride-based semiconductor layer and the second portion having difficulty in allowing growth of the first nitride-based semiconductor layer can be easily formed on the first region provided with the element, while the third portion having difficulty in allowing growth of the first nitride-based semiconductor layer can be formed on the second region.
The aforementioned first, second and third portions can be combined in the following manner: For example, the first, second and third portions may include a region having no buffer layer, a region having a buffer layer and a region having no buffer layer respectively. Alternatively, the first, second and third portions may include a region having a mask layer, a region having no mask layer and a region having a mask layer respectively. Further alternatively, the first, second and third portions may include a region having a recess portion, a region having no recess portion and a region having a recess portion respectively. Further alternatively, the first, second and third portions may include a region having a mask layer, a region having a buffer layer and a region having a mask layer respectively.
The first, second and third portions may include a region having no buffer layer, a region having a buffer layer and a region having a mask layer respectively. Alternatively, the first, second and third portions may include a region having no buffer layer, a region having a buffer layer and a region having a recess portion respectively. Further alternatively, the first, second and third portions may include a region having a mask layer, a region having a buffer layer and a region having a recess portion respectively. Further alternatively, the first, second and third portions may include a region having a mask layer, a region having no mask layer and a region having a recess portion respectively. Further alternatively, the first, second and third portions may include a region having a recess portion, a region having no recess portion and a region having a mask layer respectively. Further alternatively, the first, second and third portions may include a region having a mask layer, a region having a buffer layer and a region having no buffer layer respectively. Further alternatively, the first, second and third portions may include a region having a recess portion, a region having no recess portion and a region having no buffer layer respectively.
In the aforementioned nitride-based semiconductor element including the first, second and third portions, the first portion of the first region having difficulty in allowing growth of the first nitride-based semiconductor layer preferably includes a region having a first mask layer of a first width, the second portion of the first region easily allowing growth of the first nitride-based semiconductor layer preferably includes a region having no mask layer, and the third portion of the second region having difficulty in allowing growth of the first nitride-based semiconductor layer preferably includes a region having a second mask layer of a second width larger than the first width. According to this structure, the first portion easily allowing growth of the first nitride-based semiconductor layer and the second portion having difficulty in allowing growth of the first nitride-based semiconductor layer can be easily formed on the first region provided with the element, while the third portion having difficulty in allowing growth of the first nitride-based semiconductor layer can be easily formed on the second region.
In the aforementioned nitride-based semiconductor element including the first, second and third portions, the first portion of the first region having difficulty in allowing growth of the first nitride-based semiconductor layer preferably includes a region of a first width having no buffer layer, the second portion of the first region easily allowing growth of the first nitride-based semiconductor layer preferably includes a region having a buffer layer, and the third portion of the second region having difficulty in allowing growth of the first nitride-based semiconductor layer preferably includes a region of a second width, larger than the first width, having no buffer layer. According to this structure, the first portion easily allowing growth of the first nitride-based semiconductor layer and the second portion having difficulty in allowing growth of the first nitride-based semiconductor layer can be easily formed on the first region provided with the element, while the third portion having difficulty in allowing growth of the first nitride-based semiconductor layer can be easily formed on the second region. The first nitride-based semiconductor layer can be grown without a selective growth mask, whereby the first nitride-based semiconductor layer can be prevented from cracking resulting from the difference between the thermal expansion coefficients of the selective growth mask and the first nitride-based semiconductor layer.
In the aforementioned nitride-based semiconductor element according to the first aspect, the first region provided with the element preferably includes a first portion, having a first recess portion of a first depth, having difficulty in allowing growth of the first nitride semiconductor layer and a second portion easily allowing growth of the first nitride-based semiconductor layer, and the second region preferably includes a third portion, having a second recess portion of a second depth larger than the first depth, having difficulty in allowing growth of the first nitride-based semiconductor layer. According to this structure, the first nitride-based semiconductor layer is more difficult to be grown on the third portion of the second region provided with the second recess portion having a larger depth as compared with the first portion of the first region provided with the first recess portion having a smaller depth, whereby the thickness of the portion of the first nitride-based semiconductor layer formed on the second region can be easily reduced as compared with that of the portion of the first nitride-based semiconductor layer formed on the first region. Further, the first nitride-based semiconductor layer can be grown without a selective growth mask, whereby the first nitride-based semiconductor layer can be prevented from cracking resulting from the difference between the thermal expansion coefficients of the selective growth mask and the first nitride-based semiconductor layer.
In the aforementioned nitride-based semiconductor element including the first, second and third portions, the first portion of the first region having difficulty in allowing growth of the first nitride-based semiconductor layer preferably includes a region having a third recess portion of a first width, the second portion of the first region easily allowing growth of the first nitride-based semiconductor layer preferably includes a region having no recess portion, and the third portion of the second region having difficulty in allowing growth of the first nitride-based semiconductor layer preferably includes a region having a fourth recess portion of a second width larger than the first width. According to this structure, the first portion easily allowing growth of the first nitride-based semiconductor layer and the second portion having difficulty in allowing growth of the first nitride-based semiconductor layer can be easily formed on the first region provided with the element and the third portion having difficulty in allowing growth of the first nitride-based semiconductor layer can be easily formed on the second region. Further, the first nitride-based semiconductor can be grown without a selective growth mask, whereby the first nitride-based semiconductor layer can be prevented from cracking resulting from the difference between the thermal expansion coefficients of the selective growth mask and the first nitride-based semiconductor layer.
In the aforementioned nitride-based semiconductor element according to the first aspect, the first region provided with the element preferably includes a first portion, having a third mask layer of a first thickness, having difficulty in allowing growth of the first nitride-based semiconductor layer and a second portion easily allowing growth of the first nitride-based semiconductor layer, and the second region preferably includes a third portion, having a fourth mask layer of a second thickness larger than the first thickness, having difficulty in allowing growth of the first nitride-based semiconductor layer. According to this structure, the first nitride-based semiconductor is more difficult to be grown on the third portion of the second region having the fourth mask of a larger thickness as compared with the first portion of the first region having the third mask layer of a smaller thickness, whereby the thickness of the portion of the first nitride-based semiconductor layer formed on the second region can be easily reduced below the thickness of the portion of the first nitride-based semiconductor layer formed on the first region.
The aforementioned nitride-based semiconductor element according to the first aspect is preferably divided into each element in the second region. According to this structure, the second region easily cracked due to concentration of strain can be effectively utilized as a region for separation of the substrate into each element.
In the aforementioned nitride-based semiconductor element according to the first aspect, the second region is preferably formed to enclose the first region in plane. According to this structure, the second region capable of relaxing strain of the first region can be formed around the first region, thereby effectively relaxing strain of the first region. In this case, the second region may be provided in the form of a lattice.
In the aforementioned nitride-based semiconductor element according to the first aspect, the second region is preferably formed to hold the first region in plane. According to this structure, the second region capable of relaxing strain of the first region can be formed on each side of the first region, thereby effectively relaxing strain of the first region. In this case, the second region may be linearly formed.
The aforementioned nitride-based semiconductor element according to the first aspect preferably further comprises a nitride-based semiconductor element layer, formed on the first nitride-based semiconductor layer, having a first area. According to this structure, the nitride-based semiconductor element layer having the first area can be formed on the first nitride-based semiconductor layer inhibited from cracking and reduced in degree of warpage of the substrate, whereby a nitride-based semiconductor element layer having excellent element characteristics can be easily formed. Consequently, a nitride-based semiconductor element having excellent element characteristics can be obtained.
In the aforementioned nitride-based semiconductor element according to the first aspect, the first nitride-based semiconductor layer preferably has a larger thermal expansion coefficient than the substrate. When the first nitride-based semiconductor layer has a large thermal expansion coefficient, the portion of the first nitride-based semiconductor layer formed on the first region is easily strained. According to the present invention, the second region capable of relaxing strain can effectively relax such strain of the portion of the first nitride-based semiconductor layer formed on the first region. In this case, the substrate may include either an Si substrate or an SiC substrate.
In the aforementioned nitride-based semiconductor element according to the first aspect, the first nitride-based semiconductor layer preferably has a smaller lattice constant than the substrate. When the first nitride-based semiconductor layer has a smaller lattice constant than the substrate, the portion of the first nitride-based semiconductor layer formed on the first region is easily strained. According to the present invention, the second region capable of relaxing strain can effectively relax such strain of the portion of the first nitride-based semiconductor layer formed on the first region.
In the aforementioned nitride-based semiconductor element according to the first aspect, dislocations are preferably laterally bent thereby reducing the density of vertical dislocations in the portion of the first nitride-based semiconductor layer located on the first region. According to this structure, a first nitride-based semiconductor layer having excellent crystallinity can be obtained.
A nitride-based semiconductor element according to a second aspect of the present invention comprises a first region formed on a prescribed region of a substrate and provided with an element including a first nitride-based semiconductor layer having a prescribed thickness and a second region formed on a region of the substrate other than the first region and provided with no first nitride-based semiconductor layer. The first region provided with the element includes a first portion, formed on the substrate, having difficulty in allowing growth of the first nitride-based semiconductor layer, and a second portion, formed on the substrate, easily allowing growth of the first nitride-based semiconductor layer, and the second region includes a third portion, formed on the substrate, having difficulty in allowing growth of the first nitride-based semiconductor layer.
The nitride-based semiconductor element according to the second aspect is formed with the second region provided with no first nitride-based semiconductor layer as described above, whereby strain of the first region provided with the element is so relaxed that the portion of the first nitride-based semiconductor layer formed on the first region can be effectively prevented from cracking and the degree of warpage of the substrate can be reduced. Consequently, the nitride-based semiconductor element can be formed on the first region with an excellent yield.
In the aforementioned nitride-based semiconductor element according to the second aspect, the first portion of the first region is preferably formed with a first width, and the third portion of the second region is preferably formed with a second width larger than the first width. According to this structure, the first nitride-based semiconductor layer is more difficult to be grown on the third portion of the second region having a larger width as compared with the first portion of the first region having a smaller width, whereby the second region can be easily prevented from formation of the first nitride-based semiconductor layer.
In the aforementioned nitride-based semiconductor element according to the second aspect, the first portion of the first region having difficulty in allowing growth of the first nitride-based semiconductor layer preferably includes any of a region having no buffer layer, a region having a mask layer and a region having a recess portion, the second portion of the first region easily allowing growth of the first nitride-based semiconductor layer preferably includes any of a region having a buffer layer, a region having no mask layer and a region having no recess portion, and the third portion of the second region having difficulty in allowing growth of the first nitride-based semiconductor layer preferably includes any of a region having no buffer layer, a region having a mask layer and a region having a recess portion. According to this structure, the first portion easily allowing growth of the first nitride-based semiconductor layer and the second portion having difficulty in allowing growth of the first nitride-based semiconductor layer can be easily formed on the first region provided with the element and the third portion having difficulty in allowing growth of the first nitride-based semiconductor layer can be easily formed on the second region.
In the aforementioned nitride-based semiconductor element according to the second aspect, the first portion of the first region having difficulty in allowing growth of the first nitride-based semiconductor layer preferably includes a region having a first mask layer of a first width, the second portion of the first region easily allowing growth of the first nitride-based semiconductor layer preferably includes a region having no mask layer, and the third portion of the second region having difficulty in allowing growth of the first nitride-based semiconductor layer preferably includes a region having a second mask layer of a second width larger than the first width. According to this structure, the first portion easily allowing growth of the first nitride-based semiconductor layer and the second portion having difficulty in allowing growth of the first nitride-based semiconductor layer can be easily formed on the first region provided with the element and the third portion having difficulty in allowing growth of the first nitride-based semiconductor layer can be easily formed on the second region.
In the aforementioned nitride-based semiconductor element according to the second aspect, the first portion of the first region having difficulty in allowing growth of the first nitride-based semiconductor layer preferably includes a region of a first width having no buffer layer, the second portion of the first region easily allowing growth of the first nitride-based semiconductor layer preferably includes a region having a buffer layer, and the third portion of the second region having difficulty in allowing growth of the first nitride-based semiconductor layer preferably includes a region of a second width, larger than the first width, having no buffer layer. According to this structure, the first portion easily allowing growth of the first nitride-based semiconductor layer and the second portion having difficulty in allowing growth of the first nitride-based semiconductor layer can be easily formed on the first region provided with the element and the third portion having difficulty in allowing growth of the first nitride-based semiconductor layer can be easily formed on the second region. Further, the first nitride-based semiconductor layer can be grown without a selective growth mask, whereby the first nitride-based semiconductor layer can be prevented from cracking resulting from the difference between the thermal expansion coefficients of the selective growth mask and the first nitride-based semiconductor layer.
The aforementioned nitride-based semiconductor element according to the second aspect is preferably divided into each element in the second region. According to this structure, the second region easily cracked due to concentration of strain can be effectively utilized as a region for separation of the substrate into each element.
In the aforementioned nitride-based semiconductor element according to the second aspect, the second region is preferably formed to enclose the first region in plane. According to this structure, the second region capable of relaxing strain of the first region can be formed around the first region, thereby effectively relaxing strain of the first region. In this case, the second region may be provided in the form of a lattice.
In the nitride-based semiconductor element according to the second aspect, the second region is preferably formed to hold the first region in plane. According to this structure, the second region capable of relaxing strain of the first region can be formed on each side of the first region, thereby effectively relaxing strain of the first region. In this case, the second region may be linearly formed.
The aforementioned nitride-based semiconductor element according to the second aspect preferably further comprises a nitride-based semiconductor element layer, formed on the first nitride-based semiconductor layer, having a first area. According to this structure, the nitride-based semiconductor element layer having the first area can be formed on the first nitride-based semiconductor layer inhibited from cracking and reduced in degree of warpage of the substrate, whereby a nitride-based semiconductor element layer having excellent element characteristics can be easily formed. Consequently, a nitride-based semiconductor element having excellent element characteristics can be obtained.
In the aforementioned nitride-based semiconductor element according to the second aspect, the first nitride-based semiconductor layer preferably has a larger thermal expansion coefficient than the substrate. When the first nitride-based semiconductor layer has a large thermal expansion coefficient, the portion of the first nitride-based semiconductor layer formed on the first region is easily strained. According to the present invention, the second region capable of relaxing strain can effectively relax such strain of the portion of the first nitride-based semiconductor layer formed on the first region. In this case, the substrate may include either an Si substrate or an SiC substrate.
In the aforementioned nitride-based semiconductor element according to the second aspect, the first nitride-based semiconductor layer preferably has a smaller lattice constant than the substrate. When the first nitride-based semiconductor layer has a small lattice constant, the portion of the first nitride-based semiconductor layer formed on the first region is easily strained. According to the present invention, the second region capable of relaxing strain can effectively relax such strain of the portion of the first nitride-based semiconductor layer formed on the first region.
In the aforementioned nitride-based semiconductor element according to the second aspect, dislocations are preferably laterally bent thereby reducing the density of vertical dislocations in the portion of the first nitride-based semiconductor layer located on the first region. According to this structure, a first nitride-based semiconductor layer having excellent crystallinity can be obtained.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.